In recent years, Bluetooth (Registered trademark) communication function is used for communication between a cellular phone and a handsfree apparatus typically installed on a vehicle or the like. Bluetooth communication between the cellular phone and the handsfree apparatus is established based on a handsfree profile that serves as a communication protocol of the Bluetooth standard. In a disclosure in Japanese patent document JP-A-2004-106801, the Bluetooth communication between the cellular phone and the handsfree apparatus is enabled in a stand-by mode for immediately providing a handsfree call through the handsfree apparatus when the cellular phone receives an incoming call.
In the Bluetooth communication based on the handsfree profile, the cellular phone notifies a remaining battery level to the handsfree apparatus. Some of the handsfree apparatus is capable of regularly displaying the remaining battery level on a display window of a car navigation system for improving user's convenience.
However, the remaining battery level regularly displayed on the display window is sometimes overlooked by a user unconsciously as a matter of fact. In other words, the user has always to consciously watch the remaining battery level in order to prevent an unintended disconnection of the Bluetooth communication between the cellular phone and the handsfree apparatus. In addition, an advance warning message of battery exhaustion on a display of the cellular phone such as “Please recharge the cellular phone” or the like may not appropriately be recognized by the user when the body of the cellular phone is stored in an invisible place from of the user. Therefore, the user of the cellular phone fails to recognize the exhaustion of the cellular phone battery and to respond to the advance warning message in a timely manner.